A Microkernel has nothing to do with the size of the kernel. Rather, it refers to the functionality that the kernel provides. It is generally agreed that this is; a set of interfaces to allow processes to communicate and a way to talk to the hardware. Software drivers, as we like to call them, are then implemented in user space as servers. The most obvious examples of these are the TCP/IP stack, the ext2 filesystem and NFS. In the case of the Hurd, users now have access to functionality that, in a monolithic kernel, they could never use, but now, because the server runs in user space as the user that started it, they may, for instance, mount an FTP filesystem in their home directory.
For more information about the design of the Hurd, read the paper by Thomas Bushnell, BSG: Towards a New Strategy of OS Design.